[msvc] Allow MSVC toolchain driver to find the aarch64 / arm64 cross-compiler.

Starting with the Fall Creators Update, Windows 10 Desktop can run on
machines that are powered by aarch64 processors.

Microsoft call the aarch64 architecture "arm64". This patch maps
ArchType::aarch64 to "arm64" to allow the MSVC toolchain driver to find
the aarch64 / arm64 cross-compiler.

Patch by Chris January

Differential Revision: https://reviews.llvm.org/D44087

llvm-svn: 326744